Two general types of couplers have been developed for use in signal distribution in multiterminal data communications systems. These types of couplers are known as "star" and "tee" couplers. A star coupler can be considered as an optical mixer which links each terminal simultaneously to all other terminals in the communications system. On the other hand, a tee coupler provides at each terminal a capability for injecting or removing a signal from an optical fiber trunk line. The type and strength of that signal is variable. Certain advantages of flexibility in the number and location of the distribution paths are thus available. Moreover, in recent years low-loss tee couplers have been built and tested. These devices have demonstrated excess loss measured at about 0.2 dB, as compared with losses of previous couplers which were an order of magnitude higher.
For a more comprehensive overview of these subjects, the reader is referred to an invited paper by Mr. Thomas G. Giallorenzi published in IEEE proceedings Vol. 66, No. 7 of July, 1978 at page 761; and also to a paper entitled "Low-loss Access Coupler for Multimode Optical Fiber Distribution Networks" by Drs. B. S. Kawasaki and K. O. Hill in Applied Optics, Vol. 16, page 1794, 1977.
The former paper concludes that single mode optical fibers offer the optimum in overall propagation performance. However, he has also concluded that exploitation of this performance has been limited by the difficulty in coupling such fibers to other components and to other fibers. These difficulties are manifest, for example, in problems with alignment, and in high losses in coupling or transferring of the optical frequency signal from one fiber to another.